In an electric power steering device for applying an assist torque generated by a motor in accordance with a steering torque applied by a driver of an automobile to a steering wheel, the assist torque which is approximately proportional to the steering torque is determined so that a torque proportional gain which corresponds to an amplification factor of a proportional relationship between the steering torque and the assist torque is set large to reduce a steering force of the driver. At the same time, a vibration such as an oscillation of a control system, which is generated along with an increase in the torque proportional gain, is suppressed to improve a driver's feeling.
For the suppression of the oscillation/vibration, it has conventionally been proposed to reduce a steering frequency component of a rotation speed of a motor (hereinafter also referred to as “motor speed”) by a highpass filter (HPF) when a signal obtained by detecting or estimating the motor speed is fed back to an output torque of the motor (for example, see Patent Literatures 1 and 2). According to the above-mentioned literatures, the effects of suppressing the vibrations while suppressing the occurrence of a phenomenon in which the feedback of the motor speed becomes a resistance with respect to the steering in the steering frequency are obtained.
Moreover, as another example, it has been proposed to set a control cycle at the time of feedback control using the motor speed shorter than a cycle of an uncomfortable vibration to reduce the steering frequency component by using a bandpass filter (BPF) (for example, see Patent Literature 3). A center frequency of a selected frequency of the bandpass filter lies in the vicinity of 20 Hz to 40 Hz. Therefore, only a vibration component of a frequency band in the vicinity of the center frequency is selectively transmitted. Therefore, a signal component having a relatively low frequency as high as about several hertzes corresponding to the steering frequency does not pass through the bandpass filter. With the configuration described above, the effects of suppressing the vibrations while preventing a feeling of viscosity (viscous resistance) with respect to the steering from increasing to an undesirable level are obtained.